Macrophages (M0) and thymus derived lymphocytes (T-cells) from the spleen or peritoneal cavity of mice immune to Listeria monocytogenes (LM) collaborate with each other and release a product (lymphokine) nonspecifically cytotoxic for B-16 melanoma. Peritoneal adherent cells or M0, however, 1) do not collaborate with splenic T-cells and in fact suppress the synergistic interaction of this cell with its complementary (splenic) M0. Therefore two models of T cell/M0 interaction are observed leading to either suppression or "promotion" of tumor cells in vitro. The mechanism(s) of such cellular interactions and immunoregulation by M0 are unclear. Since a large number of immunostimulants such as BCG and C. parvum used in immunotherapy also induce "suppressor macrophages", the current studies are directed towards understanding the role of these cells in the immune response and nonspecific immunotherapy in particular. BIBLIOGRAPHIC REFERENCES: Youdim, S.: Resistance to Tumor Growth Mediated by L. monocytogenes. Destruction of Experimental Malignant Melanoma by LM Activated Peritoneal and Lymphoid Cells. J. Immunol. 116: 579, 1976. Youdim, S.: Destruction of Experimental Malignant Melanoma by Mediators of Cellular Immunity. Cancer Research 37: 572, 1977.